1. Field of the Invention
The present invention relates to an image pickup apparatus mounted to a small information terminal or the like.
2. Description of the Related Art
In recent years, small camera modules have been mounted in compact portable data handsets such as cellular phones and PDAs (Personal Digital Assistant). An optical system for such a small camera module must be small and particularly thin. To this end, in many cases, a unifocal lens system is used for the optical system to reduce thickness and a trimming (digital zooming) function is used to achieve a zooming effect. According to this trimming function, virtual telescopic photography (enlargement photography) can be carried out by trimming the angle-of-view (image capture range). Alternatively, a thin optical zoom lens system which does not utilize a trimming function has been proposed. This known optical zoom lens system is provided with at least one movable lens group which is moved to change the focal length to thereby vary the angle of view.
However, in a trimming system using a unifocal lens system, an optical image formed by the unifocal lens system is converted into a digital image (original image), so that at least a part of the digital image is extracted and enlarged in order to be displayed. Consequently, the enlarged digital image has fewer image pixels than that of the original image (original digital image which has not been enlarged). Therefore, if the extracted image is enlarged to the same size of the original image, the extracted image becomes undesirably rough. The image quality and the magnifying ratio for indicating an enlarged image are interrelated, so that the image quality is significantly reduced as the magnification is increased.
In a trimming system using an optical zoom lens, since it is necessary to provide a lens drive mechanism such as a motor or a cam in order to at least substantially continuously move the movable lens group, the lens drive mechanism tends to be complicated and large. An increase in the size of the optical zoom lens is contrary to the aim of miniaturizing and reducing the weight of the information terminal to which the optical zoom lens is to be mounted. Furthermore, in an optical zoom lens system, the movement of the movable lens group significantly deteriorates the performance due to decentering (eccentricity) thereof or the like. In particular, such deterioration in the optical performance become more apparent as the optical zoom lens system is further miniaturized. In order to solve these problems, it is possible to use a bifocal optical system in which a wide-angle photographing mode and a telephoto mode can be selected. However, in a bifocal optical system, only two focal lengths can be selectively used, and hence, the bifocal optical system is not necessarily satisfactory for a zoom lens system.
To solve the above-mentioned problems, it is possible to combine optical zooming, which is performed by a focus switching optical system, with trimming, which is performed by an image processing operation. However, if an object image at each focal length of the focus switching optical system is used and an object image at an intermediate focal length between the selectable focal lengths of the focus switching optical system is complemented by a trimmed image, a angle-of-view at which the trimmed image cannot be obtained exists when the focal length of the focus switching optical system is changed, so that a continuous zooming effect cannot be obtained.